Food starch technology

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Food starch technology [cover photo] National researchers utilize advanced instrumentation, such as computerized image analyzers and scanning electron microscopes, to determine starch granule size

1 Food starch technology [cover photo] National researchers utilize advanced instrumentation, such as computerized image analyzers and scanning electron microscopes, to determine starch granule size and distribution and to analyze surface morphology. Food starch technology a global commitment National Starch and Chemical Company is the worldwide leader in specialty starch technology and production. With technical service facilities in over 20 countries on five continents, National is constantly striving to develop new food starches with new characteristics for its customers around the globe. We back up our commitment by building state of the art starch production facilities, investing in advanced analytical equipment, expanding our food pilot plants and maintaining an expert staff of chemists and technologists. It is a fully integrated process that also encompasses the genetic breeding of our own seed corn to develop starch characteristics the marketplace demands.

2 What follows is a brief introduction of the technical knowledge obtained in the development of food starches and their applications. We have examined the sources and manufacturing processes, the chemical and physical structure and the phenomenon of gelatinization. We have also explained the reasons for modification, specifically crosslinking and stabilization. Finally, we discuss application technology - interaction of ingredients, processing and equipment. Introduction In nature starch is available in an abundance surpassed only by cellulose as a naturally occurring organic compound. It is found in all forms of green leafed plants, located in their roots, stems, seeds or fruits. Starch serves the plant as food for energy during dormancy and germination. It serves similar purposes for man and animal as well as lower forms of life. Man, however, has found uses for starch that extend far beyond its original design as a source of biological energy. Practically every industry in existence uses starch or its derivatives in one form or another. In foods and pharmaceuticals starch is used to influence or control such characteristics as texture, aesthetics, moisture, consistency and shelf stability. It can be used to bind or to disintegrate; to expand or to densify; to clarify or to opacify; to attract moisture or to inhibit moisture; to produce short texture or stringy texture, smooth texture or pulpy texture, soft coatings or crisp coatings. It can be used to stabilize emulsions or to form oil resistant films. Starch can be used to aid processing, packaging, lubrication or moisture equilibration. Starch truly serves as a multifunctional ingredient in the food industry. Sources of Starch The most common sources of food starch are corn, potato, wheat, tapioca and rice. Corn is cultivated in warmer climates, with half of the world s production grown in the U.S.A., its biggest crop. China, the second largest producer in the world, grows about 10%. Approximately 70% of the world s potato supply is grown in the cool, moist climate of Europe and Russia. Wheat, requiring a more temperate climate, is primarily grown in the former USSR, North America and Europe. Approximately 90% of world rice production comes from South and Southeast Asia, while tapioca is cultivated in the narrow tropical band about the equator. Wet Milling of Corn Corn starch is extracted from the kernel through a process of wet milling. The process employs techniques of grinding, screening and centrifugation to separate purified starch from fiber, oil and tightly bound protein. (Fig. 1).

The wet milling process begins with softening of the kernel by steeping it in a dilute acid solution. Coarse grinding splits the kernel to remove the oil-containing germ.

3 The wet milling process begins with softening of the kernel by steeping it in a dilute acid solution. Coarse grinding splits the kernel to remove the oil-containing germ. Finer milling separates the fiber from the endosperm which is then centrifuged to separate the less dense protein from the more dense starch. The starch is then washed and dried or left in a slurry for further processing, such as cross-linking. Structure The building blocks of carbohydrates are α-d and β-d glucose which contain six (6) carbon atoms and form pyranose rings. Through enzymatic condensation, one molecule of water is split out between two molecules of glucose to form a bond. This condensation occurs predominantly between carbons 1 and 4 (Fig. 2) but occasionally between 1 and 6.

* (*Traditionally, amylose is considered as being only linear in configuration, but recent investigations indicate the presence of limited branching

4 Where only the α-1,4 linkage develops, a linear chained homopolymer results which we refer to as amylose. (Fig.3). The length of this chain will vary with plant source but in general the average length will run between 500 and 2,000 glucose units.* (*Traditionally, amylose is considered as being only linear in configuration, but recent investigations indicate the presence of limited branching in some amylose molecules. However, for the sake of simplicity and better understanding of the properties of amylose, this presentation will ignore those findings and consider amylose as linear only.)

It is interesting to note that amylose and cellulose are very similar in structure with the single exception of the spatial arrangement of the bridging between the numbers 1 and 4 carbons.

5 It is interesting to note that amylose and cellulose are very similar in structure with the single exception of the spatial arrangement of the bridging between the numbers 1 and 4 carbons. The beta glucose form found in cellulose results in a rigid molecule with strong intermolecular bonding which is not digestible by humans. The alpha linkage of amylose allows it to be flexible and humanly digestible. The second type of polymer in starch develops when the enzymatic condensation between glucose units occurs at carbons 1 and 6. This occasional linkage, along with the predominant 1,4 bonding, results in a branching effect and the development of a molecule much more massive in size than amylose but with linear chain lengths of only glucose units. This molecule is called amylopectin. (Fig. 4). All starches are made up of one or both of these molecules, but the ratio of one to the other will vary with the starch source. Corn has about 25-28% amylose with the remainder being amylopectin. High amylose corn can run as high as 80%. Tapioca has about 17% amylose, and waxy maize has virtually none. As might be predicted from this data, the cooked characteristics of tapioca lie somewhere between those of corn and waxy maize, and the characteristics of corn are greatly accentuated in high amylose corn. The Starch Granule As the plant produces the starch molecules, it deposits them in successive layers around a central hilum to form a tightly packed granule. Wherever possible, adjacent amylose molecules and outer branches of amylopectin associate through hydrogen bonding in a parallel fashion to give radially oriented, crystalline bundles known as micelles. These micelles hold the granule together to permit swelling in heated water without the complete disruption and solubilization of the individual starch molecules. (Fig.5).

6 These highly oriented and crystalline micellular areas explain the ability of ungelatinized starch granules to rotate a plane of polarized light to produce characteristic interference crosses. This bi-refringent cross is one of the features used in identifying starch source. When the radial orientation of the crystalline micelle is disturbed, the bi-refringent cross disappears. (Fig. 6).

Gelatinization Gelatinization temperatures are considered as ranges covering the temperatures at which loss of bi-refringence is first noticed and less than 10% remains.

7 Gelatinization Gelatinization temperatures are considered as ranges covering the temperatures at which loss of bi-refringence is first noticed and less than 10% remains. This temperature range is greatly influenced by the binding forces within the granule which vary with species. High amylose corn has much greater bonding force than the other maize varieties due to the high degree of linearity within the granule. On the other hand, orthophosphate ester groups within the potato granule tend to weaken bonding and lower energy requirements to gelatinize.

8 When the starch granule is heated in water, the weaker hydrogen bonds in the amorphous areas are ruptured and the granule swells with progressive hydration. The more tightly bound micelles remain intact, holding the granule together. Birefringence is lost. As the granule continues to expand, more water is imbibed, clarity is improved, more space is occupied, movement is restricted and viscosity increased. With the swelling of amylose-containing granules such as corn, the amylose molecules are solubilized and leach out into solution. These molecules will then reassociate into aggregates and precipitate at low concentrations or set to a gel at higher starch concentrations. This is referred to as set back or retrogradation. The congealed paste will become cloudy and opaque with time and will eventually release water to shrink into a rubbery consistency (Fig. 7). Waxy maize has essentially no linear amylose molecules so its paste will remain flowable and clear. It will not gel or weep. Tapioca, having a small amount of amylose, gives a soft gel when pasted. Pastes from high amylose starch set to a very stiff gel. To summarize the physical changes during gelatinization: the granule swells and loses birefringence; clarity and viscosity increase; and smaller linear molecules (if present) dissolve and reassociate to form a gel. Brabender Amylographs To follow the viscosity behavior of starch while cooking, we use the Brabender Viscoamylograph. The starch suspension is heated in a revolving cup to a designated temperature and held there, as the viscosity is measured through resistance exerted on a stirrer suspended in the pasted starch. A continuous recording of this resistance is marked on a moving chart. The resulting graphical curve records the point of gelatinization, the rate of viscosity development, the peak viscosity and the rate of viscosity breakdown. If desired, further viscosity information can be recorded during the cooling cycle. Corn starch heated in water to 95 C will show a rather rapid increase in viscosity after gelatinization until it reaches a peak. The viscosity will gradually decrease during the holding period, then dramatically increase again as the paste cools and retrogrades. Waxy maize will increase in viscosity at a more rapid rate than regular corn. The peak viscosity for waxy maize will be greater and will be attained sooner. However, it will also break down in viscosity faster and to a greater extent. On cooling, waxy maize shows little increase in viscosity because it does not gel. (Fig. 8).

9 Potato starch will absorb more water, showing a high initial peak viscosity, when compared to other native starches. The gelatinization temperature is lower, causing the solution to thicken quickly on heating. The high peak viscosity falls rapidly during this holding period. The solution shows little tendency to retrograde on cooling. Tapioca starch gelatinizes at a temperature between that of waxy maize and corn, and has a slightly lower viscosity than waxy maize. The cooled solution retrogrades to produce a soft gel. Solutions of cooked tapioca are characterized by a greater clarity when compared to other native starches. Why Modify Starch? In the unmodified form, starches have limited use in the food industry. Waxy maize starch is a good example. The unmodified granules hydrate with ease, swell rapidly, rupture, lose viscosity and produce weak bodied, very stringy and very cohesive pastes. We modify starch to enhance or repress its inherent properties as appropriate for a specific application: to provide thickening, improve binding, increase stability, to improve mouthfeel and sheen, to gel, disperse or cloud. Cross-linking We cross-link to control texture and to provide heat, acid and shear tolerance. As a result, we have better control and improved flexibility in dealing with formulation, processing and product shelf-life. Cross-linking of starch can be thought of as a means to spot weld the granule at random locations, reinforcing hydrogen bonding and inhibiting granule swell. (Fig. 9).

This cross-linking treatment strengthens the relatively tender waxy starches so that their cooked pastes are more viscous and heavy bodied and are less likely to breakdown with extended cooking

10 This cross-linking treatment strengthens the relatively tender waxy starches so that their cooked pastes are more viscous and heavy bodied and are less likely to breakdown with extended cooking times, increased acid or severe agitation. The change in the swelling characteristics is obvious from the Brabender curves of unmodified and lightly modified waxy maize. With very light modification the viscosity breakdown is reduced dramatically. With a moderate level of treatment, the granule swell is restricted to the point where peak viscosity is never reached during the holding period. As the cross-linking reaction progresses, the peak viscosities first increase and then decline to very low values as the swelling of the starch granules become progressively more inhibited. At the same time, shortness of string and opacity of paste both increase. (Fig. 10). As the degree of cross-linking is increased, the starch becomes more tolerant to acid and less likely to break down. This is not to suggest that the most highly cross-linked starch will give the

11 best viscosity in low ph food systems. It must be remembered that cross-linking inhibits granule swell whereas high temperature, extended heating, high hydrogen ion concentration and high energy input all tend to disrupt hydrogen bonding and enhance granule swell. With this in mind, one should select the starch which is cross-linked sufficiently to withstand chemical and physical abuses and still give maximum viscosity. If it happens that a moderately cross-linked starch tends to break down when cooked at low ph, the problem can sometimes be solved with a procedural change. By cooking the starch at a higher ph, allowing the paste to cool and then adding the acid to reach the desired ph, a satisfactory viscosity may be reached without changing to a more highly cross-linked starch. Stabilization Another important starch modification is that of stabilization. This modification prevents gelling and weeping and maintains textural appearance. Earlier, it was mentioned that the linear fraction of cooked corn starch will reassociate through hydrogen bonding causing gelling, opacity and weeping. Since the highly branched waxy variety has no amylose, it will not retrograde or gel under normal storage conditions. However, under low-temperature or freezing conditions, a waxy paste will become cloudy and chunky and will weep, somewhat like the paste made with regular corn starch. This is attributed to a lowering of kinetic movement as the temperature drops, allowing the outer branches of the waxy starch to associate through hydrogen bonding and bringing about similar but less dramatic conditions that occur with amylose. To prevent the occurrence of this condition, anionic groups are scattered throughout the granule to block molecular association through ionic repulsion as well as steric hindrance. (Fig. 11). The result of this treatment is a stabilized starch which will produce pastes that will withstand several freeze-thaw cycles before syneresis (weeping) occurs. Freeze-thaw stabilized starches are essential to the frozen food industry but have applications in many other areas as well. Cold temperature storage conditions of other processed foods such as canned sauces and gravies are commonplace and require stabilized starches to maintain quality.

12 Effects of Ingredients The ingredients present during the cooking cycle have very definite effects on the swelling characteristics of the starch and the finished viscosity of the paste. Acids disrupt hydrogen bonding to bring about more rapid swelling of the granule. Soluble solids interfere by tying up the water necessary for hydration. Fats and proteins tend to coat starch which delays granule hydration and lowers rate of viscosity development. The ph of the medium is very important in determining the proper starch selection. An unmodified starch will peak earlier and break down faster at a ph of 2.5 than it will at a ph of 4.0 (Fig. 12). Cooking starch in the presence of high concentrations of soluble solids can also present problems. Soluble solids, such as sugars, compete for the water required to hydrate the starch granule to allow it to swell. Using lightly inhibited or even pregelatinized starches will help overcome the problem, but usually the best approach is to hold back most of the sugar or other soluble solids until after the starch has been completely cooked. Adding solids at this point will give the desired viscosity at the desired solids. (Fig. 13).

Effects of Time, Temperature and Shear In selecting the proper starch to do the job, one must also consider the processing temperature, the length of time at that temperature and the forces of shear

13 Effects of Time, Temperature and Shear In selecting the proper starch to do the job, one must also consider the processing temperature, the length of time at that temperature and the forces of shear that the pasted starch will encounter. The higher the temperature, the greater the shear and longer the time exposed to these forces, the more swollen the granule and the more fragile and susceptible it is to rupture. We can build in tolerances to shear, temperature and acid by supplementing hydrogen bonds in the granule by cross-linking. (Figs. 14 & 15).

14 Under- and Over-cooked Starch Pastes It is essential to reach gelatinization temperature during processing to ensure that the texture benefits of the starch are realized. The two exceptions to this rule are the use of pregelatinized starch and the use of starches in cook-up mixes, where the consumer will adequately cook the mix at home. When foods are heat treated to pasteurization temperature (75 C), unless the proper starch is selected, the starch paste in the food system may be under-cooked, resulting in a cloudy, thin product. If foods are held at elevated temperatures for extended times, as may be the case in a kettle cook prior to filling, the starch may be over-cooked. This may result in a product with an undesirably, long cohesive texture. If the heat processing temperature of a food system cannot be raised without risking damage to certain delicate ingredients, then under-cooking of the starch may be rectified by pre-cooking a starch slurry before blending in other ingredients. Over-cooking of starch may be avoided by using a more cross-linked, process tolerant starch. (Fig. 16).

15 Impact of Processing on Starch Selection The impact of processing equipment on the starch granule cannot be overemphasized. Shearing forces exerted by high speed mixing, milling, homogenization or pumping can damage the starch granule. (Figs ). As mentioned earlier, by cross-linking the starch we can build in tolerance to shear as well as to temperature and acid. This is an absolute requirement for salad dressing starches which are cooked at low ph, at high temperatures and are also subjected to colloid milling. Pudding starches subjected to flash cooling would be another example of a need for shear tolerance.

16 Processing Equipment Steam jacketed kettles with sweeping mixers are normally considered as low in shear; plate cookers as medium in shear; steam injection cookers, plate and flash coolers and milling equipment as high, and homogenizers as extremely high in shear. This is a general statement, and it is possible that a steam jacketed kettle may do as much damage to the starch granule over an

17 extended time as a homogenizer in short time. If it is necessary to homogenize a starchcontaining food product such as a pudding, it is common practice to homogenize before the starch is cooked, as the suspended uncooked granules will pass through the homogenizer unharmed. (Fig. 20) National Starch and Chemical Company

Starch Applications in Meat Products. Kris J. Swenson Tom Katen

Starch Applications in Meat Products Kris J. Swenson Tom Katen Effect of Non-Meat Ingredients Salt Phosphate Starch Addition of Starches Method of addition Type of product Further processing Storage conditions